Pneumatic tires have traditionally been constructed by applying an outer homogenous tread stock over a supporting carcass structure and vulcanizing the resulting composite structure. An outer matrix of grooves is molded or otherwise provided in the outer portions of this homogenous tread stock to provide traction as well as other desirable characteristics.
Certain tread compounds provide better traction than others. Also, certain tread compounds provide better rolling resistance than others. Although a tire which has both low rolling resistance and a maximum amount of traction is desirable, a tread compound which provides good rolling resistance in a tire may not generally provide a maximum amount of traction, and a tread compound which provides a maximum amount of traction may not provide as low of rolling resistance as may be desired.
Pneumatic tires having a tread/running surface comprised of a silica-rich rubber composition, while sometimes desirable to impart various physical properties to the tire tread such as, for example, reduced rolling resistance and suitable traction, may be disadvantageous because of the relatively high ratio of silica to carbon black in the reinforcing filler content. Such high silica/carbon black ratios represent a significant increase in cost of the silica-rich tread in terms of increased material cost (the silica) and increased cost of processing the silica-rich rubber composition. Further, such silica-rich tread rubber, with its minimal carbon black content, may have a relatively low electrical conductivity and may therefore be electrically resistive to conducting static electrical charge from the tire tread to the ground.
A path of increased electrical conductivity for a silica-rich tread may be provided, for example, by positioning a strip of a carbon black rich rubber composition either as a thin cover strip over a portion of the running surface of the tread or as a thin, non load bearing, strip extending through the body of the tread to its running surface. Methods of such type add both cost and complexity to the tire itself and to the manufacturing procedure for the tire.
Some tire treads have a cap/base construction, with the tread cap designed to be ground-contacting with a lug/groove configuration, and with the tread base underlying and supporting the tread cap and positioned between the tread cap and the tire carcass/belt structure. The tread base is not intended to be ground-contacting and, thus, not normally intended to have the same tread properties as, for example, the desired tread cap properties of traction and treadwear.
While the tread cap, in a tread cap/base construction, may be designed to be ground-contacting and, therefore, provide traction in combination with acceptable tread wear and rolling resistance, the underlying tread base may be designed to fulfill an entirely different function and not be designed to be ground-contacting at all. In particular, the tread base may fulfill a function of transmitting multiaxial tread cap forces to the tire carcass, usually with relatively low heat generation. These forces may include forces resulting from the tread cap working under forces such as compression, bending, and/or shear, all of which may generate heat, cause temperature build-up, and cause the forces to undesirably impact on the tire carcass itself. Such forces may result, for example, from the tire's cornering, braking, and/or varied handling activities, all of which may generate heat within the tire tread.
In one conventional pneumatic tire, in order to both reduce the material and fabrication cost of a silica-rich tread and also provide a path of increased electrical conductivity from the tire through its tread to the ground, a tread running surface may be divided into three distinct load bearing zones, which may include at least one silica-rich load bearing zone and at least one carbon black-rich load bearing zone of rubber compositions. By requiring the tread cap zones to be load-bearing, each of the three distinct running surface tread cap zones may extend from the outer surface of the tread to the underlying distinct carbon black-rich tread base rubber layer so that all of the load on the tire is communicated by each of the three tread cap layer zones directly to the tread base layer instead of directly to the tire carcass itself.